The present invention relates to an optical processing unit for parallel operation, which incorporates an optical memory function. In the prior art, optical methods of data processing have been mainly utilized for filtering processing, such as image enhancement, noise elimination, etc, or for correlation processing based on matched filters. Such processing takes a substantial amount of time to perform, if electronic signal processing means are employed. However optical processing using a single lens and the Fourier transform can be employed to perform such processing by parallel two-dimensional operation at the speed of light. There have also been proposals for employing two-dimensional parallel optical processing to perform computation which has hitherto been performed by electronic means. For example, by using an array of LEDs, a matrix mask and an array of photodectors, it is possible to carry out matrix computations by optical processing. Such matrix computation by optical means has been described in detail in an article entitled "Microprocessor-Based Fiber Optic Iterative Processor" which appeard on pages 147 to 152 of the journal "Applied Optics", of January 1982. As descried in this article, matrix compuation is performed utilizing a linear array of LEDs, a planar spatial modulator (for example a hologram), and a linea array of photo-detectors. With that method, matrix multiplication processing is performed optically by two-dimensional parallel processing, and so can be carried out at very high speed. However, no optical memory means is incorporated, so that it is necessary to store intermediate results of the iterative processing in some form of electrical memory in order to enable successively generated intermedaite results to be compared with one another. In addition, due to the fact that optical processing and electrical processing are directly combined, it is necessary to provide elements for performing conversion from optical to electrical signals, and vice-versa. Due to the time necessary to perform these conversion operations, the speed of processing is substantially lower than that which is possible if purely optical processing is carried out.
It has also been proposed to employ an optical logic element made up of an optical gate array, i.e. a planar array of optical gate elements. Details of such optical logic elements are given in an article entitled "Digital Optical Computing" apearing in the Proceedings of the IEEE, in July 1984 (pages 758 to 779). The article describes how it is possible to form an optical logic system for sequential processing by employing a planar array of optical gate elements and a planar optical interconnection unit. The proposed arrangement is illustrated in FIG. 1 of the drawings, and is made up of an optical gate element array 14 and a reconfigurable optical interconnection unit 15. The optical gate element array 14 can be considered to consist of three sections, i.e. an input/output section 11, in central processing unti (CPU) 12, and a memory section 13. The I/O section 11 employs light valves for input and output of data. I/O section 11, CPU 12 and memory section 13, are mutually interconnected in an arbitrarily selectable arrangement, through the reconfigurable optical interconnection unit 15, by light beams 16. In addition, the optical gate elements in CPU 12 can access any arbitrarily selectable optical gate element of I/O unit 11 or memory section 13. In this way, a non Von Neumann type of processor can be implemented by optical means. If the optical gate elements are utilized and NAND gates, then a general sequential type of flip-flop circuit can be configured with this sytem. Utilizing liquid crystal light valves for the optical gate element array 14, and a computer-generated hologram for interconnection unit 15, a practical system having this configuration was set up and tested, as described in the above article. Since the optical gate elements are connected by loops, formed of light beams, it is possible to define any desired logic function by employing an appropriate combination of interconnections and gate elements. However, in order to implement a wide range of processing capabilities it is necessary for the optical interconnection unti to be capable of changing the interconnections formed thereby at very high speed, to thereby successively implement different logic functions. In practice it is extremely difficult to attain a sufficiently high speed of interconnection switching, i.e. to employ elements which will perform such switching at a speed which will be compatible with the operating speed capabilities of optical processing. This is a basic disadvantage of such an optical logic system.